That isn't currently possible, unfortunately:
A fairly old feature request for plan templates to reuse across projects (BAM-907) has been resolved as Fixed due to the introduction of plan branches in Bamboo 4.0 (see Using plan branches for details):
Plan Branches are a Bamboo Plan configuration that represent a branch in your version control system. They inherit all of the configuration defined by the parent Plan, except that instead of building against the repository's main line, they build against a specified branch. It is also worth noting that only users with edit access to the Plan can create Plan Branches that inherit from that plan.
While plan branches are a killer simplification for typical Git workflows around feature branches and pull requests indeed and might help accordingly, they neither fully cover the original request nor yours, presumably - that aspect is meanwhile tracked via Add possibility to create plan templates and choose a template when creating a plan (BAM-11380) and esp. Build and deployment templates (BAM-13600), with the latter featuring a somewhat promising comment from January 2014:
Thank you for reporting this issue. We've been thinking about templates a lot over the last few months. When we've got more news to share on this, we will be sure to update this ticket.